First Online: 07 October 2013Received: 02 July 2013Accepted: 01 October 2013

Abstract

The longstanding assumption that calves of more than 6 months of age are more resistant to Mycobacterium avium subspecies paratuberculosis MAP infection has recently been challenged. In order to elucidate this, a challenge experiment was performed to evaluate age- and dose-dependent susceptibility to MAP infection in dairy calves. Fifty-six calves from MAP-negative dams were randomly allocated to 10 MAP challenge groups 5 animals per group and a negative control group 6 calves. Calves were inoculated orally on 2 consecutive days at 5 ages: 2 weeks and 3, 6, 9 or 12 months. Within each age group 5 calves received either a high – or low – dose of 5 × 10 CFU or 5 × 10 CFU, respectively. All calves were euthanized at 17 months of age. Macroscopic and histological lesions were assessed and bacterial culture was done on numerous tissue samples. Within all 5 age groups, calves were successfully infected with either dose of MAP. Calves inoculated at < 6 months usually had more culture-positive tissue locations and higher histological lesion scores. Furthermore, those infected with a high dose had more severe scores for histologic and macroscopic lesions as well as more culture-positive tissue locations compared to calves infected with a low dose. In conclusion, calves to 1 year of age were susceptible to MAP infection and a high infection dose produced more severe lesions than a low dose.

AbbreviationsBHIBrain heart h infusion

HDHigh dose

HPCHexadecylpyridinium chloride

HEHematoxylin-Eosin

JDJohne’s disease

LDLow dose

LNLymph node

MAPMycobacterium avium subspecies paratuberculosis

ZNZiehl-Neelsen.

Electronic supplementary materialThe online version of this article doi:10.1186-1297-9716-44-94 contains supplementary material, which is available to authorized users.